Better Lasing With Pulses

The folks at the Lansing, Michigan hackerspace built themselves a 40 Watt laser cutter. It’s an awesome machine capable of cutting plywood and acrylic, and is even powered by a RAMPS board, something normally found in 3D printers. They wanted a little more power out of their 40 Watt tube, though, and found pulsing the laser was the best way to do that.

Unlike the fancy Epilog and Full Spectrum Laser machines, the Buildlog.net 2.x laser cutter found in the Lansing Hackerspace didn’t use Pulse-Per-Inch (PPI) control until very recently. When a laser tube is turned on, the output power of the laser is much higher – nearly double the set value – for a few milliseconds. By pulsing the laser in 2-3 ms bursts, it’s possible to have a higher effective output from a laser, and has the nice added benefit of keeping the laser cooler. The only problem, then, is figuring out how to pulse the laser as a function of the distance traveled.

To do this, the laser cutter must accurately know the position of the laser head at all times. This could be done with encoders, which would require a new solution for each controller board. Since laser cutters are usually driven by stepper motors controlled with step and direction signals, a much better solution would be to count these signals coming from the CNC computer before it goes to the RAMPS driver, and turn the laser on and off as it moves around the bed.

A few tests were done using various PPI settings, each one inch long, shown in the pic above. At 200 PPI, the laser creates a continuous line, and at higher PPI settings, the lines are smoother, but get progressively wider. The difference between PPI settings and having the laser constantly on is subtle, but it’s there; it’s not quite the difference between an axe and a scalpel, but it is a bit like the difference between a scalpel and a steak knife.

It’s an impressive build for sure, and something that brings what is essentially a homebrew laser cutter a lot closer to the quality of cutters costing thousands of dollars. Awesome work.

Are you sure? When an electrical device over-volts to kick off a process, and you force it to undergo that process much MUCH more frequently, the result is normally that it’s lifetime drastically reduces.

“One disadvantage is the sputtering of the metal electrode material in contact with the
discharge. These DC electrodes require periodic replacements because of wear resulting from the sputtering away of the electrode material caused by bombardment from energetic ions within the discharge. Another disadvantage is that the sputtering of the electrode material into the discharge contaminates the laser gas, thereby shortening the laser’s lifetime. The sputtering also enhances the formation of arcs within the discharge, which reduces the quality of the laser beam. Still another disadvantage is the high electric fields required to maintain the discharge.”

So BattleTech’s pulse lasers actually had some merit? That’s actually very interesting.
(http://www.sarna.net/wiki/Large_Pulse_Lasers, though they generated more heat, not less, and worked by dissipation of the material vapor.)

Pulsing a laser like this is called ‘Q-switching’, where Q stands for beam quality. A low quality beam makes the laser stop its stimulated emission and instead ‘charge up’ energy in electron orbits as the power supply remains on.

…I can’t find any reference to the beam modulator they used in the article. Are they actually switching the HV power supply on-and-off? They can’t get pulses shorter than 2ms max that way since the atoms take a while to discharge fully.
I wonder if they have not mistaken how the commercial units perform this operation. If so it’s amazing that they got any results at all, and that their computational constraints arrived at just the exact numbers of the upper performance range of this unusual mode of operation.

I wonder if commercial units don’t use another laser system entirely? CO2 isn’t really meant for pulsed operation (the recovery time of the gas is pretty slow and the gain spectrum is quite narrow). Something like YAG is much more suited to pulsed operation, and peak powers of each pulse are much much MUCH higher. Even at low repetition rates (<50Hz) the peak power can approach something like several kilowatts for pulsewidths in the nanosecond range. You can cut tons of stuff with YAG. ;)

I’ve been devoted to studying CO2 lasers these past, nearly two months. I’m familiar with the theory by now but not as much with using the jargon properly… Everybody seems to have their own preference in lasing medium!

CO2 lasers are Q-switched at 80 – 100 kHz in commercial units. The highest rate I have seen in the research paper abstracts I’ve been able to dig up is 120 kHz. There is an experiment I wish to run, but it requires absorption in mid-IR, several Watts of power RMS, and a sustained pulse rate of about 1 MHz (!).

I think building such an AOM can be done. If not, at least I have my laser build as an entry in the Hackaday prize contest. =)

Very interesting indeed. I’m not all that familiar with gas systems, mostly solid-state. Do you have any references related to this you could send or post? I’d be interested to learn how it’s done in gas systems, since it isn’t necessarily trivial.

We have AOM based pulse pickers here which are suitable for 1 micron light and can go as high as 10 MHz, so I don’t see why it wouldn’t be possible with longer wave IR (depending on material properties, etc.. as I’m sure you already know, things get rather tricky for mid-IR stuff).

Choice of gain medium depends on application – I’m not overly familiar with why one would choose CO2 over YAG in an industrial setting, especially now days when you can easily get 100kW fiber systems (typically 1 micron wavelength, both pulsed and CW) the size of a dishwasher with minimal cooling and maintenance requirements.

The Q in Q-factor refers to the ‘quality factor’ of the actual optical resonator cavity. In true q-switched systems, the q-switch introduces high losses and prevents lasing (i.e. active feedback from the end-cavity mirrors) to allow energy to build up in the gain medium. This generates an excess of population inversion in the gain medium, storing much more energy than in the CW (continuous wave) state. Theoretically, the gain is higher when q-switched (but only for a short time) since under CW operation, the gain and losses must be equal. This setup is commonly used with pulsed laser systems.

The beam quality is something different entirely.

The setup they’re using here is *not* q-switching. While in theory, they’re (sort-of) modulating the gain and population inversion, really they’re just relying on the fact that the tube is always slightly over-voltage when starting (in order to initiate plasma), so the output power is higher.

As for tube lifetime it is possible that it will be shortened drastically unless the tube is designed for this kind of operation. With multiple stops and starts at relatively high voltage, I seem to remember that you run the risk of sputtering the electrodes, which ultimately affects the operation of the tube. I’m not entirely sure though – this topic definitely needs more research. ;)

I’ve never had an issue with anything you have wrote on here before but “Unlike the fancy Epilog and Full Spectrum Laser machines” is daft. FS is not fancy, they’re re-badged Chinese machines with a new board(I also have an issue with the patenting others work). Try Trotec, ULS or even G Weike but never FS.

not quite. There are many Chinese laser engraver/cutter manufacturers, and each has its own quality standard. There Is a limited number of designs for the case, so all chinese lasers tend to look the same, especially the cheapest ones. I’ve heard it said though that most of them do not take the cheapest model very seriously, it is the larger models that get the better engineering etc.
Full spectrum may be importing a cheap chinese model, they do also sell a design I have yet to elsewhere. That one has aluminium extrusion + bearings for the XY movement, with a lens that can move in the Z direction. All mounted in a sturdy case and controlled with decent software (which is usually the big problem with cheap lasers). We use this laser for demos etc on ;location where the small size/weight is a big plus.. For around 5000 euro’s delivered it was not a bad buy I think.

My comments only applies to the 40W models, not the bigger stuff (that FS also sell). That said FS doesn’t seem to do a lot of work on the bigger ones either. (Someone could make a tidy sum selling decent mirror mounts.)

FS have gotten hold of a redesigned version of the 40W – 5th Gen – that doesn’t appear to be popular yet.

There’s actually a smaller version as well that pops up from time to time.

The comercial units I repaired (1.5KW Co2), had also a pulsed mode but the laser was always on and the pulse get up to 125%, it was a mix of DC and pulses. Usually used to cut copper, because of it’s high reflection, but many mirror and lenses died because of these reflections.

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